Movable dam and method of operation

ABSTRACT

A MOVABLE DAM FOR NAVIGABLE WATERCOURSE OR LAKE IN WHICH A STATIONARY PORTION OF THE DAM IS ANCHORED IN THE BED OF THE WATER COURSE AND A MOVABLE GATE SYSTEM SERVES TO REGULATE THE CROSS-SECTIONAL AREA FOR FLOW OF WATER THROUGH THE DAM.

27, 1973 J. AUBERT v v MOVMBLE DAM AND METHOD OF OPERATION F1104 July 27, 1971 3 Sheets-Sheet 1 I I; 7 L; M. a

Feb. 27, 1973 J. AUBERT MOVABLE DAM AND METHOD OF OPERATION 5 $heets-Sheer2 Filed July 27, 1971 Feb. 27, 1913 J, AUBERT 3.718902 MOVABLE DAM AND METHOD OF OPERATION Filed July 27, 1971 3 Sheets-Sheet's United States Patent Office U.S. Cl. 61-22 13 Claims ABSTRACT OF THE DISCLOSURE A movable dam for a navigable watercourse or lake in which a stationary portion of the dam is anchored in the bed of the water course and a movable gate system serves to regulate the cross-sectional area for flow of water through the dam.

The movable gate system comprises on the one hand at least one heavy closure element which can be adjusted under load and at least one light closure element which can be adjusted only in the dry and the sill of which is located at a height greater than the value corresponding to the depth of free navigation at the level of the dam when the heavy closure element has been fully withdrawn.

This invention relates to a movable dam which is intended to be built across waterways of 'dilierent types such as large or small rivers and spillway-lakes, especially for the purpose of regulating the water level at predetermined locations upstream of the structure. This invention is also concerned with the method of operation of a dam of this type.

It is known that the movable dams which have been developed up to the present time for watercourses such as rivers and the like usually comprise a fixed portion of masonry which is anchored in the bed of the watercourse and placed across this latter. The fixed portion usually has an opening-or channel-in which is mounted a movable portion or so-called gate. Depending on its position with respect to the fixed portion, the gate system serves to regulate the cross-sectional area which is provided for the flow of water through the aforesaid channel. As a 'rule, the movable gate system is so arranged as to ensure that the bed of the watercourse is completely freed in the open position.

In general, the gate system comprises at least one movable closure element which can vary Widely in structural design. The closure element can in fact be constituted by a pivotally mounted or sliding gate or alternatively by a shutter and the dam can comprise a series of movable closure elements such as a line of shutters, for example.

The aim of movable dams of this type is to retain water when the rate of flow is of low value in order to ensure a minimum depth under all circumstances (for example, the depth which is necessary for navigation) and in a predetermined position which is located upstream of the dam.

On the contrary, when the rate of flow of the watercourse is of high value, the dam must be opened while completely freeing the movable gate system in order to otter no resistance to the flow of water since this would have the effect of producing an undesirable increase in the upstream flood level.

In structural designs which are at present known, the closure element or elements of the movable gate system can be operated not only when their upstream and downstream faces are at the same pressure (in the position of total immersion or emersion) but also when they are under a water load and serve to retain the water, in which case their upstream and downstream faces are subjected to very diiterent pressures.

These operations are essential since the greater or lesser degree of opening or closure of the gate system must be 3,718,002 Patented Feb. 27, 1973 adapted to the volume of water admitted on the upstream side or, in the case of a river, to the flow rate of this latter. Said flow rate is continuously variable at all times of the year over a fairly wide range.

In practice, the possibility of regulating the cross-sectional area which is left free for the flow of water through the movable gate system is ensured in order to maintain the minimum water level required for navigation at a predetermined point located upstream of the dam despite seasonal and random variations in flow rate of the watercourse.

The need to ensure that the movable gate system can be operated under load over its full range of travel results in particularly rugged designs provided with powerful operating means. These devices entail high capital outlay which increases the overall cost of the dam.

The aim of this invention is to provide a remedy for this situation without thereby affecting the possibility of regulating the cross-sectional area for flow through the movable gate system and is primarily directed to the construction of movable dams of economical design, that is to say dams which are appreciably lower in capital cost than a conventional movable dam having the same retaining capacity.

In accordance with the invention, the movable dam for watercourses has a fixed portion which is anchored in the bed of the watercourse and a movable gate system for regulating the cross-sectional area provided for the flow of water through the dam and is distinguished by the fact that the movable gate system comprises on the one hand at least one first closure element which can be regulated under load or so-called heavy closure element and at least one second closure element which can be operated only in the dry" or so-called light closure element.

Since the cost price of the light closure element is substantially lower than that of the heavy closure element and the same applies to the operating system employed for said light element, the cost price of the dam can be reduced in proportion to the area represented by the light closure element or elements with respect to the total area of the gate system.

In accordance with a preferred embodiment of the invention, the height at which the sill of the light closure element is located is greater than that which corresponds to the height of free navigation along the watercourse between the dam and a predetermined upstream position, the heavy closure element being assumed to be fully withdrawn.

In accordance with a first embodiment, the light closure element and the heavy closure element are juxtaposed in side-by-side relation whereas, in a second embodiment, said two elements are superposed, the sill of the light closure element being located above the channel which is provided for the displacement of the heavy element.

Since the light closure element is intended to be operated only in the dry and preferably has only two positions, namely a position of full withdrawal and a position of maximum retention, the constructional design both of the light closure element and of its retaining and operating system can be appreciably simplified.

Thus, the light closure element can be constructed by means of a shutter which is pivotally mounted on a horizontal pin forming part of the shutter sill and provided with a retaining prop associated with a slide, said shutter being provided with only two operating positions, namely a substantially vertical retaining position and a substantially horizontal position of withdrawal.

Also in accordance with the invention, the method for operating light and heavy closure elements constituting the movable gate system in a dam of the type which is contemplated in the foregoing is distinguished by the fact that the light closure element is operated in the dry since the sill of said element is not reached by the water level at the moment of operation.

In a first mode of execution of the method under consideration, the light closure element of the movable gate system being open, closure of said element is carried out when the flow rate of the watercourse falls below a value which is equal to the flow rate for natural navigation between a predetermined upstream position and the dam when the gate system of this latter is fully open.

In a further embodiment, the light closure element which was previously in the closed position is fully opened when the flow rate of the watercourse exceeds a value which is higher than the rate of flow for natural navigation between a predetermined upstream position and the dam but without permitting said flow rate to reach a value at which the water level at the dam attains the sill of the light closure element when the heavy closure element has been withdrawn.

Experience has shown that, if the two conditions mentioned above are complied with, a suflicient period of time which is nevertheless compatible with the provision of simplified operating means can accordingly be allowed for the operation of the light closure element.

Further particular features of the invention will be brought out by the description which now follows, reference being made to the accompanying drawings which are given by way of non-limitative example, and in which:

FIG. 1 is a view in elevation showing a dam of a simplifi'ed type in accordance with the invention;

FIG. 2 is a sectional view along line 11-11 of FIG. 1;

FIG. 3 is a sectional view along line 'IIIIII of FIG. 1;

FIGS. 4, 7, 10 and 13 are diagrammatic views which are similar to FIG. 1 and show the dam in different positions of adjustment;

FIGS. 5, 8, 11 and 14 are views which are similar to FIG. 2 and show the corresponding positions of adjustment of the heavy closure element;

FIGS. 6, 9, 12 and 15 are views which are similar to FIG. 3 and show the corresponding positions of adjustment of the light closure element;

FIG. 16 is an explanatory sectional diagram showing the bed of the watercourse upstream of the dam shown in FIG. 1;

FIG. 17 is a view which is similar to FIG. 1 and shows a movable dam comprising a number of light closure elements and one heavy closure element;

FIG. 18 is a diagrammatic sectional view of a movable dam in which the light closure element is superposed on the heavy closure element;

FIG. 19 is a view in side elevation showing a light shutter and of the foundation raft which is shown in transverse cross-section;

FIG. 20 is a perspective diagram of the aforesaid shutter;

FIG. 21 is a view in perspective and on a larger scale showing a portion of FIG. 20.

Reference will now be made to FIGS. 1 to 3 of the accompanying drawings which show a movable dam in accordance with the invention. Said dam comprises a stationary portion 1 of masonry which is anchored in the bed 2 of the watercourse. The movable dam is intended to regulate the height of that portion of said watercourse which is located between position A of the dam and a predetermined position B which is located upstream (as shown in FIG. 16), especially in order to ensure that the depth at B should be at least equal to the value h under all circumstances (namely the minimum value of free navigability, for example). The position B can correspond in particular to the downstream portion of another movable dam.

In accordance with the present invention, the movable gate system of the darn comprises two types of closure elements: a first so-called heavy element 3 mounted in a channel formed in the masonry 1 and a second 4 so-called 1ight" closure element 4 mounted within another channel 6 of the masonry 1, the channels 5 and 6 being substantially juxtaposed in the example under consideration.

The channels 5 and 6 have different depths, the sill 7 of the channel 5 being located at a greater depth than the sill 8 of the channel 6. The depth of said channel 6 is determined by certain conditons which will be explained hereinafter. If the reference S designates the area of the channel 5 which can be closed by means of the closure element 3 and the reference S designates the area of the channel 6 which can be closed by means of the closure element 4, steps are taken to ensure that the area S +S is comparable with that which corresponds to natural filling of the profile B of the watercourse bed 2 at the level of the dam at the moment of maximum high water.

The channel 5 for the heavy closure element is preferably formed in that portion of the profile P which has the greatest depth whilst the channel 6 for the light closure element 4 is formed in that portion of the profile P which has the smallest depth so that the assembly consisting of both channels 5 and 6 is naturally inscribed within the profile P.

According to one important feature of the invention, the sill 8 of the channel 6 is formed above a water level having the reference H at A which corresponds to the flow rate Q, of the water course at which natural navigation becomes possible along the entire length of the channel between positions A and B (depth of water at least equal to h at B) assuming that the closure elements 3 and 4 are fully open, in which case the dam does not have any appreciable action on the water levels. This entails the need to ensure that the cross-sectional area S of the channel 5 is sufiicient to permit the flow Q to pass under the conditions given above. The entire area S of the movable gate system (namely the system corresponding to the light closure element 4) is in that case located above the level H The height H of the sill 8 of the channel 6 is located in practice at a level which is higher than H by a value m which corresponds as will become apparent later to a safety margin which increases the range of operation of the light closure element 4.

By heavy closure element 3 is meant in this description any movable member for closing the channel 5 which can be operated under load, that is to say the height of which can be adjusted whilst the upstream face is subjected to the hydrostatic pressure and the downstream face of said member is free. A closure element of this type can be constituted in a conventional manner by a sliding or pivoting gate or by one or a number of rocking shutters. An operating mechanism of any known type is associated with said heavy closure element 3 and serves to effect the height adjustment of said element under load. By way of example, said mechanism can consist of lifting chains or jacks or alternatively in the case of shutters of an articulated operating carriage which is capable of moving above the shutters.

By light closure element 4 is meant in this description any movable closure member which can be operated only in the dry, that is to say when both faces of said member are not subjected to any pressure difference. Said closure element can also be formed by a sliding or pivoting gate or by one or a number of shutters. Since it is intended that the water should never pass above the shutter 4 when this latter is in the closed position and that said shutter should not be in contact with the streams of water when it is in the withdrawn position, its structure can be made appreciably less rugged than that of the heavy closure element 3 both in regard to the plating and the structural members which support said plating. This structure is therefore much less costly.

As a complementary feature, the closure element 4 is intended to be brought to only two positions, namely a fully open position in which said element is completely withdrawn and a fully closed position in which the channel 6 is completely closed. Since the closure element 4 is operated in the dry and always takes up the same position at the end of operation, namely either the fully open or fully closed position, the operating means can be highly simplified. They can consist of handling means which are external to the dam (e.g., a crane or truck-mounted crane) or alternatively of means which are stationarily mounted on the dam but have low power (winches and jacks in particular).

Another economy factor lies in the fact that the masonry 1 which limits the channel 6 is never exposed either to currents or to shocks produced by the flow of water under pressure.

Further economy factors arise from the following considerations:

the light closure element 4 is frequently in the dry state, thus permitting the use of a less massive and rugged structure which can be readily repainted;

there is no danger of any vibrations which are liable to be produced 'by a liquid flowing at high velocity and, more generally, the dynamic actions of the water are eliminated;

any danger of impacts from debris which are carried by high-velocity streams of water (ice fragments or even boats) is removed.

All these factors serve to simplify and lighten the structure of the dam and of the movable gate system over the above-mentioned area S The description which now follows will relate to the method of operation which is advantageously provided for the light closure element 4 as a function of the variation in flow rate Q of the watercourse.

The notations employed hereinafter will be understood to have the following meanings:

Q the flow rate corresponding to the maximum flood discharge which is contemplated;

and H the level which corresponds to the dam and which is substantially the same both on the upstream and downstream sides since the gate system is in that case open;

Q the very low flow rate at which the gate system of the dam is practically horizontal;

and H the water level which has to be maintained upstream of the dam by means of a virtually complete closure of the gate system in order to ensure the depth h at the critical point B. Since the water surface is horizontal in this case, the level H is higher by the distance h than the level of the channel floor at the critical point B.

It will be assumed that the positions A and B are relatively distant from each other and that it has consequently been found necessary at the level of the dam under consideration to accept a water level H which is higher than the water level H Q an intermediate flow rate between Q and Q and H the level which prevails at the dam and which is the same both on the upstream and downstream sides since the dam is fully open;

Q,-: a flow rate comprised between Q and Q and H the height which must be maintained on the upstream side of the dam by partial closure of the gate system in order to ensure a depth h at the critical point B.

Whatever type of closure element is employed, it will be assumed in order to illustrate the method which is contemplated by the invention that the variable and partial opening of the channel corresponds to a reduction in height of the heavy closure element 3 and that the flow takes place above the portion which is maintained in position within the channel 5.

When the flow rate is equal to Q or to a closely related value, the dam is in the condition shown in FIGS. 1 to 3. The water flows freely through the channel 5 and the heavy closure element 3 is completely withdrawn (as shown in FIG. 2). Since the sill 8 of the closure element 4 is at the level H and higher than the level H by a value m, said closure element. is entirely out of the water; there is no flow of water through the channel 6. The closure element 4 is in that case fully withdrawn (as shown in FIG. 3).

Above the flow rate Q the filow of the watercourse will vary progressively in a random manner.

If the flow rate increases and assumes a value Q which is higher than Q (as shown in FIGS. 4 to 6), no operation is necessary: the water continues to flow freely through the channel 5. When the level H, reaches the level Hg, the light closure element 4 in the fully withdrawn position is again covered. In this position, the closure element 4 is not subjected to any stress. If the flow continues to increase and attains the value Q of maximum flood discharge, no operation is necessary since the two channels 5 and 6 are filled with water which flows at the level H, (as shown in FIGS. 7, 8, 9).

It is therefore evident that, if the closure elements 3 and 4 are withdrawn in the case of the flow rate Q,,, no operation is necessary in the event of a rise to flood level If the flow rate falls below the value Q it is first necessary to close the heavy closure element 3 in order to maintain the level h at the upstream position B. In the case of a value Q, of the flow rate at which the water level is at H, below H full closure of the light closure element 4 is then carried out in a single operation (as shown in FIGS. 10 to 12). The closure element 4 is then again out of the water.

If the flow rate continues to fall, the closure element 3 is fully closed (as shown in FIGS. 13, 14, 15) and the top edge of this latter comes to the same level as the edge of the closure element 4. The water level rises to H and is retained partly by said closure element 3 and partly by the light closure element 4 which has previously been placed in the raised position. The closure element 4 is not subjected to any overflow and it is thus apparent that the structure and foundation of this latter can be highly simplified.

The height m between the level H and the level H therefore corresponds to a safety margin which allows the required time interval for raising the light closure element 4. As this height is smaller, so the area S is larger and so the cost price of the structure as a whole is consequently lower. On the other hand, the time interval which is permitted for raising the light closure element 4 is accordingly reduced in proportion.

The safety margin will be greater as the light closure element 4 is of more simple constructional design and is operated by hand, for example. Even in the case of a light closure element 4 of the mechanically operated type, the operation can be carried out in advance even if it may subsequently prove necessary to perform the reverse operation if the flow rate does not vary in the expected direction. In fact, taking into account the light construction of the closure element 4, an operation entails only low power consumption.

The method is thus characterized by the following points:

the light closure element 4 is usually open;

closure of said element is then carried out and is ac-- cordingly complete when the flow rate attains a value Q which is appreciably lower than Q if the closure element 4 is closed, it must be fully opened before the flow rate has exceeded a value Q, which is higher than Q As is clearly brought out by the description of the method which is claimed, the fact that the light closure element can neither be opened not closed under load 7 does not constitute any hindrance whatever so far as concerns the maintenance of a correct adjustment of the water level upstream of the dam.

If the rates of fiow of the watercourse vary frequently and rapidly, it will prove an advantage to employ a mechanical operating device for the light closure element 4.

The following numerical values which are given by way of example serve to define the invention.

n the basis of a total gate area S=3684 m? in the case of a large river, provision can be made for a series of light closure elements representing an area S =2100 111. namely more than one-half the area of the gate system. In point of fact, the cost per square meter of that part of the dam which is equipped with light closure elements is five times lower than the cost per square meter of the heavy closure element.

In this example, the level H =2.5 m. and the level H is assumed to be equal to 3 m. The selected value of flow rate Q corresponding to withdrawal of the light closure elements is equal to 1400 m. /sec. and the selected value of flow rate Q,- for raising the light closure elements is equal to 600 mfi/sec. In practice, the height H =3 m. is attained under flood conditions in respect of a flow rate of 1980 m. /sec. and under low-water conditions in respect of a flow rate of 440 mfi/sec.

The values of flow rates which are chosen for initiating operation of the light closure elements therefore make it possible to ensure that:

said closure members are opened before the flow rate attains 1980 m. /sec.;

said closure members are closed before the flow rate drops to 440 mfi/sec.

In fact, a time interval of several days is allowed be fore it becomes necessary to carry out an operation which requires only a few hours to perform.

Referring now to FIG. 17, there is shown a particular embodiment of a movable dam according to the invention comprising a single heavy closure element 3 and a series of light closure elements 4a, 4b, 4c, and so forth which are in aligned and in juxtaposed relation to the heavy closure element 3. This arrangement is particularly well suited to a river having a bed which is relatively narrow under normal flow conditions but become considerably wider under fiood conditions.

In the embodiment of FIG. 18, the first heavy closure element or elements (for example of the butterfly valve type) are shown at 13 and housed in channels forming ducts 15 in the masonry 1. The ducts 15 are surmounted by the raft 18 and this latter serves to support the second light closure element or elements 14 which may be of the shutter type, for example, the props of which are shown at 20. The maximum retention level is shown in the figure at H One advantageous form of construction of a second light closure element is illustrated in FIGS. 19 to 21. This closure element comprises a shutter 21 which is pivotally mounted on horizontal pins 22 of the masonry sill 8. The shutter 21 comprises a sheet steel panel 23 reinforced by two upstream sectional members 24 braced by beams which are not shown in the figure. Back struts or props 25 are pivotally attached to the upright members 24 at the mid-height of these latter; the head of each prop is attached to said upright members by means of a yoke 26 and a pin 27 as shown in FIG. 21. The base 28 of the prop 25 is applied against a slide 31 of a particular type known as a Pasquaud hurter. The slide 31 comprises a stop groove 32 which serves to maintain the shutter 21 in the top position. There is formed upstream of the groove 32 a disengaging groove 33 which is shaped obliquely towards the upstream side and downwards and tends to guide the base 28 in a lateral passageway 34 which is limited on one side by a guide flange 35.

Hydraulic control means are additionally provided for ensuring operation of the shutter 21 in the dry. These means comprise a jack 36 which is pivotally mounted on a pin 37 arranged at the bottom of a masonry pit 38 which is formed between the two slides 31. The sliding rod 39 of the jack 36 terminates in a lifting beam which is pivotally mounted at 42 in bearings 43 which are attached to the upright members 24. The means for providing a connection between the shutter 21 and the foundation raft further comprise chains 44 which are attached at one end to the upright members 24 and at the other end to the foundation raft 1 by means of springs 45.

When the shutter (or shutters) 21 which closes one of the channels 6 is in the bottom horizontal position (shown in chain-dotted lines in FIG. 19) and when it is desired to raise said shutter, oil under pressure is delivered into a common pipe which supplies the jack or jacks 36. The lifting movement is stopped by the chains 44 as soon as the bases 28 of the props 25 take up positions located between the stop grooves 32 and the disengaging grooves 33. When the pressure is allowed to fall, all the props are applied against the stop grooves 32.

By reason of the fact that the operation is carried out in the dry, it is sufiicient to make use of cylinders of very small diameter which are wholly similar to those fitted on dump trucks or lorries. If the same operation were to be carried out against the water pressure, a different form of construction would be necessary and the economic performance of the system would be appreciably reduced.

In order that the shutters 21 which are in the raised position and applied against their props 25 may then be lowered to the horizontal position, oil under pressure is again delivered into the jacks 36 but this time under a higher pressure. Under this action, the elongation of the springs 45 of the retaining chains 44 is sufiicient to ensure that each prop passes beyond the disengaging groove 33 at the upstream end. As the pressure drops, so all the panels are lowered to the horizontal position, the bases 28 slide towards the downstream end along the troughs 34 and are guided by the flanges 35.

By way of indication, since the light shutter 21 is only operated in the dry and is subjected to the static pressure of the water, the weight of said shutter can be assumed to be equal to 2 tons in respect of a height of 3 m. and a width of 4 In. On the contrary, the weight of a heavy shutter having the same operating area under load would be of the order of 6 tons.

What I claim is:

1. A movable dam for watercourses such as rivers, lakes and the like, having a fixed portion which is anchored in the bed of the watercourse and a movable gate system for regulating the cross-sectional area for the flow of water through the dam, the movable gate system comprising a first and a second closure element, the sill of the first closure element being located at a level lower than the level of the sill of the second closure element, and means for moving the first closure element between open and closed positions relative to a portion of said cross-sectional area when only the upstream face of said first closure element is subjected to a water pressure.

2. A dam according to claim 1, wherein the height at which the sill of the second closure element is located is greater than that which corresponds to the height of free navigation along the watercourse between said dam and a predetermined upstream position when the first closure element has been fully opened.

3. A dam according to claim 1, wherein the second closure element and the first closure element are juxtaposed in side-by-side relation, the top edges of said elements being located at the same level in the closed position.

4. A dam according to claim 1, wherein the second closure element and the first closure element are superposed, the sill of the second element being located above a channel in which the first element is movable.

5. A dam according to claim 1, wherein the second closure element is located in that region of the watercourse bed which has the least depth, and the first closure element is located in the region of the watercourse bed which has the greatest depth.

6. A dam according to claim 1, wherein the second closure element comprises a shutter which is pivotally mounted on a horizontal pin and has at least one retaining prop that slides in a slide, said shutter having only two operating positions, namely a substantially vertical retaining position and a substantially horizontal open position.

7. A darn according to claim 6, wherein the shutter of the second closure element is directly pivoted about a horizontal pin which forms part of the sill of said shutter.

8. A dam according to claim 6, wherein the angular position of the shutter is controlled by a jack which is housed in a pit located downstream of the sill and which is pivotally mounted at the bottom of said pit.

9. A dam according to claim. 6, wherein the downstream face of the shutter is connected to said fixed portion by means of an elastic restoring member and wherein the base of said prop is mounted for movement along said slide, said slide having a disengaging groove upstream of a stop groove for maintaining the prop and the shutter in the closed position.

10. A dam according to claim 6, wherein said shutter has two props associated with two slides between which is formed a pit containing a pivotally mounted lifting jack.

11. A method for regulating the flow of water through a darn disposed in a watercourse such as a river, a lake and the like, by regulating a cross-sectional area for the flow of water through the dam, comprising the steps of positioning in said cross-sectional area a movable gate system comprising a first and a second closure element with the sill of the first closure element located at a level lower than the level of the sill of the second closure element, moving the first closure element between open and closed positions relative to a portion of said crosssectional area with only the upstream face of said first closure element subjected to the pressure of water in said watercourse, and moving said second closure element to a closed position relative to a portion of said cross-sectional area prior to the time that the water level reaches the sill of said second closure element whereby said second closure element is moved to closed position when out of contact with water in said watercourse.

12. A method according to claim 11, wherein the second closure element of the movable gate system being open, closure of said second closure element is carried out when the flow rate of the watercourse falls below a value which is equal to the tlow rate for natural navigation between a predetermined upstream position and the darn when the gate system is fully open.

13. A method according to claim 11, wherein the second closure element which was previously in the closed position is fully opened when the flow rate of the watercourse exceeds a value which is higher than the rate of flow for natural navigation between a predetermined upstream position and the dam but Without permitting said flow rate to attain a value at which the water level at the darn reaches the sill of the second closure element when the first closure element has been opened.

References Cited FOREIGN PATENTS 120,687 6/ 1927 Switzerland 6l22 764,042 12/ 1956 Great Britain 6125 1,387,316 12/1964 France 6l-8 JACOB SHAPIO, Primary Examiner U.S. Cl. X.R. 619, 25, 26 

